Fix class types to PascalCase in flutter-freezed

[oriollpz]

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Description

Classes are created PascalCase, but they are called snake-cased, I fixed that by calling the classes PascalCase as well.

Related # (issue)

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as
  expected)
• This change requires a documentation update

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How Has This Been Tested?

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• Unit test cases
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Test Environment:

• OS: MacOS
• @graphql-codegen/flutter-freezed:
• NodeJS:

Checklist:

• I have followed the
  CONTRIBUTING doc and the
  style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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[Parables]

@oriollpz Thank you for your contribution to this plugin.

Cheers 🍻